1. Field of the Invention
This invention is related to electrostatically operated micro-optical devices and method of manufacturing such devices.
2. Description of the Prior Art
Considerable interest has recently been shown in optical microelectromechanical systems (optical MEMS) based on comb drives using an electrostatic actuation scheme. Combining the comb-drive actuator with the silicon-on-insulator (SOI) and deep-reactive-ion-etching (DRIE) process, many MEMS based components have been demonstrated such as optical switch, variable optical attenuator (VOA), and Fourier transform spectrometer (FTS), etc. It can be seen that, for example, W. Noell, et al., “Applications of SOI-Based Optical MEMS”, IEEE J. on Selected Topics in Quantum Electronics, Vol. 8, No. 1, January/February 2002, pp. 148–154; C. Marxer, et al., “A Variable Optical Attenuator Based on Silicon Micromechanics”, IEEE Photonics Technol. Lett., Vol. 11, No. 2, 1999, pp. 233–235; C. Marxer and N. F. de Rooij, “Micro-Opto-Mechanical 2×2 Switch for Single-Mode Fibers Based on Plasma-Etched Silicon Mirror and Electrostatic Actuation”, IEEE J. of Lightwave Technology, vol. 17, No. 1, 1999, pp. 2–8; W.-H. Juan and S. W. Pang, “High-Aspect-Ratio Si Vertical Micromirror Arrays for Optical Switching”, IEEE J. Microelectromechanical Systems Vol. 7, No. 2, 1998, pp. 207–213. Prior arts of U.S. Pat. No. 6,315,462, “Fiber Optic Circuit Switch and A Process for Its Production,” O. Anthamatten and C. Marxer; and U.S. Pat. No. 6,229,640, “Microelectromechanical Optical Switch and Method of Manufacture Thereof,” N. Zhang have described the utilization of DRIE and wet etching release process technologies to construct the optical switch devices from SOI wafer, or bonded silicon wafers. Such disclosed micro-optical devices comprise a high-aspect-ratio micro-mirror with vertical sidewall and an electrostatic comb drive actuator for controlling the position of micro-mirror. The common comb drive actuator includes a stationary comb finger electrode, and a movable comb finger electrode connected with the movable part, i.e., the micro-mirror in this case, via a suspended spring. This mentioned suspended spring is anchored on to substrate at one end. The electrostatic force for moving the micro-mirror can be generated by applying voltage to comb drive actuator. The restoration force generated by the deformed spring will pull the actuated micro-mirror returning to the initial position. Regarding to the application of optical switch, micro-mirror can be moved from the initial off-state (light transmission state) to the actuated on-state (light reflection state, i.e., switching) via applying voltage to comb drive actuator. On the other hand, the light attenuation range for VOA application is determined in terms of the in-plane position of Si micro-mirror, where this in-plane position is controlled via force balance between electrostatic force and spring force. Thereby it can control relative amount of attenuation by blocking part of light beams.
It is important for micro-optical devices like optical switch and VOA devices to be operated at low electrical power consumption in order to keep the overall power consumption of the whole optical network system as low as possible. As a result, the electrostatic actuation scheme offered by MEMS actuator shows no power consumption and is the best candidate for optical switch and VOA applications, because there no electrical current flows through electrodes. However, a continuously applied electrical load on MEMS actuator is necessary to hold the micromirror of optical switch staying at the on-state, because we need the force generated by MEMS actuator to balance the restoring force from spring. Therefore, mechanically-bistable mechanisms, i.e., latch mechanism, providing two relative positions that are both mechanically stable is desirable for optical switch to maintain at on-state without power consumption. Prior art of U.S. Pat. No. 6,303,885, “Bi-stable Micro Switch” B. P. Hichwa, C. Marxer, and M. Gale has disclosed a latched optical switch using buckled-beam with the arch-shaped leaf spring geometry driven by a bi-directional movable electrostatic comb actuator. Additionally, prior art of U.S. Pat. No. 6,549,107, “Latching Mechanism for MEMS Actuator and Method of Fabrication” M. Lim, R. Fan, and L. Que has disclosed the other kind of latch mechanism for optical switch has been realized by using gripper to clamp the switch at one position.
It is also important for micro-optical devices like optical switch and VOA devices to have low insertion loss, low polarization dependent loss, and low back reflection loss for practical applications. Combining the MEMS elements with micro-optics provides optical switch and VOA devices a free-space light path design approach. This is a key way to make the light beam coming from input fiber become collimated beam shape thereby to gain in better optical performances. The larger collimated beam size, from several tens to hundreds of micrometers, will make better optical performance, and make the acceptable alignment tolerance higher. However, it will also lead to a requirement that the corresponding MEMS actuator has to be able to provide enough displacement to let micromirror fully block or reflect the incoming light beam. In the conventional design of electrostatic comb-drive actuator, the maximum static displacement of comb actuator is limited by the side sticking effect of comb fingers. The tiny deviations of comb finger and gap width will cause the unbalanced force of both sides of finger electrode, and such deviation is easily induced by microfabrication process. The unbalanced force of both sides of finger electrode is the major contribution factor to the side sticking effect. How to design and make a comb drive actuator that is more robust to the process induced deviation and provides longer displacement is very attractive to industrial pratical uses. There are two fundamental approaches to realize a comb drive actuator with such capability, one is making the spring perpendicular to mirror moving direction as stiffer as possible, and the second one is making the force output of comb drive as higher as better.
The present invention provides micro-optical devices with electrostatic microactuator for optical switch and variable optical attenuator devices applications, and its relative manufacturing process techniques. According to aforementioned functional requirements for applications of optical switch and variable optical attenuator devices, the desirable design of an electrostatic microactuator should include large displacement, large force output, latch mechanism, design-in mechanisms to gain in better optical performance, and design-in processes and device structures to gain in better production yield, and so on. Therefore, die present invention discloses an electrostatic microactuator, and related structures and manufacturing processes especially emphasizing in fulfilling such design requirements.